Unfortunately, TagEditor doesn't let you make amendments to files in the same way you would in a Word document, i.e. typing straight in. Instead you have to open each segment you want to edit using Workbench, just as you do when translating. It's infuriating, but saves you deleting some potentially important links embedded in the text. Hope that helps.
Thank you, that wasn't quite clear to me after reading the manual - it is now working. Still a lot to learn ..